ST PETERSBURG, Russia (Reuters) - Russia and China have agreed on a $25 billion prepayment under a supply deal signed earlier this week, Alexander Medvedev, chief executive of Gazprom Export, said on Friday.

"A prepayment has been agreed, agreed in principle, details are being discussed. It's $25 billion," Medvedev told journalists on the sidelines of the St Petersburg International Economic Forum.

Russia and China signed a 30-year gas supply contract, worth a total of more than $400 billion, earlier this week during President Vladimir Putin's visit to the country.